1. Field
The present invention relates to optical communications, and more particularly, to a coherent receiver.
2. Description of the Related Art
With the gradual enhancement on the requirements of capacity and flexibility of the optical communications system, the coherent optical communication technology has become more and more important. In comparison with incoherent technology (such as on-off key, OOK) or auto coherent technology (such as differential quadrature phase-shift keying, DQPSK), the coherent technology has the following advantages: optical signal-to-noise ratio (OSNR) gain of 3 dB; the capability to make convenient use of equalization technology; and the capability to use more efficient modulation technologies (such as quadrature modulation, QAM). Like the case in electric coherent technology, an optical coherent receiver also requires a device to recover the phase of a carrier. With the development of technologies concerning electronic devices, more and more digital technologies are employed in optical communications to solve problems that are hard to be solved in the optical field of technology. Dany-Sebastien Ly-Gagnon et al. introduce in OFC2005 OTuL4 an optical coherent receiver that makes use of digital signal processing technology. They use forward phase estimation based on digital signal processing to replace the optical phase-locked loop that is practically difficult to realize. FIG. 1 illustrates an optical coherent receiver existed in prior art. As shown in FIG. 1, an optical mixer 102, a local laser oscillator 103, photoelectric detectors 104, 105 and analog-to-digital converters (ADC) 106, 107 make up a front end processing section 118 of the coherent receiver. The front end processing section 118 converts an optical input signal 101 into a base band digital electric signal I+jQ108 (hereafter referred to also as base band electric signal), where I is a cophase component and Q is a quadrature component. Since there is no phase-locked loop, the base band electric signal 108 contains not only data information but also a phase error between the carrier and local oscillation. A phase error estimator 109 estimates the phase error and outputs an estimated value 113. An argument calculator 110 is used to obtain the argument of a complex input, namely to obtain a phase 111 of the base band electric signal 108, wherein the phase 111 is a summation of the data phase and the phase error. A subtracter 112 subtracts the phase error 113 from the phase 111 to obtain a phase data 119. A data recovering section 114 finally outputs a recovered data value. The phase error estimator 109 consists of a quadruplicater 116, an averager 117, an argument calculator 115 and a divided-by-four section 120 arranged in series to one another. The signal rate can be as high as 40 GHz in a high-speed optical communications system. Such a high-speed signal puts a very high demand on the computational capability of the digital signal processing hardware in the receiver. From another point of view, the computational capability of currently available digital signal processing hardware also restricts the implementation and application of optical digital coherent receivers. The phase error estimator of existing methods contains quadruplicate computation on complex numbers. The complexity of such multiplying computation is considerably higher than addition, subtraction and logical computations.
In view of the foregoing circumstances, there is an urgent need for a simplified phase error estimating method, particularly for one without multiplying computation so as to reduce the demand on the processing capability of the digital signal processing hardware.